Xerath is a champion with some long range, who can push the wave and has some good zone damage which can really make a difference in a teamfight if he manages to do all his spells and hit the carry, with fatal consequences. However despite all his qualities he lacks an escape and is very weak.

Xerath Counters

Ahri

Ahri is a very strong champion against Xerath because she is very good at pushing her lane and also has some sustain thanks to her passive which can be used against Xerath's poke. She is also able to poke him if he gets too close and comes into range.

The biggest advantage that Ahri has against Xerath is that she has some good mobility so she can avoid his Arcane Barrage (R) or even his Arcanopulse (Q) thanks to her Spirit Rush (R) which allows her to be aggressive at the same time and trade without any problems.

However be careful not to get stunned because that way she can deal you a lot of damage, and make sure you can see the AoE of Xerath's ultimate before using your own so you can be sure to avoid it.

Fizz

Fizz is a champion with some huge damage, and some good mobility thanks to his Playful / Trickster (E) which allows him to avoid spells from Xerath which are usually skillshots, giving Fizz the chance to see the projectile arriving and use his E in defence.

He also has the advantage of being able to easily aggress Xerath with his Urchin Strike (Q) coupled with his Seastone Trident (W) which does some very good damage. He can then escape with his E or continue to play aggressively if Xerath has used some of his cooldowns while Fizz was in the air.

Fizz will have to use the little window when Xerath uses his Locus of Power (W) when Xerath is immobile to do his Chum the Waters (R) and then use his burst.

Gragas

Gragas is a champion who is naturally quite tanky and can take Xerath's poke while being able to avoid some of his damage with his Body Slam (E).

He can also push the wave well thanks to his Barrel Roll (Q) which also allows him to poke Xerath and he can take the advantage by moving to another lane and allowing his team to snowball.

The main advantage that Gragas has over Xerath is that he can use his Explosive Cask (R) to prevent Xerath from benefiting from his Locus of Power (W) and also put Xerath in a dangerous situation which could be fatal considering he has no escape.